Crohn's disease is a chronic inflammatory disease involving the gastrointestinal tract. In man, the inflammatory cytokine TNF alpha is expressed in the bowel wall in areas of inflammation due to Crohn's disease and is thought to be a key inflammatory mediator in this disease. CDP571 is able to neutralize human TNF alpha. This trial will determine whether administration of CDP571 has potential as a treatment for patients with Crohn's disease.